The invention relates to a digital phase-locked loop (PLL), and more particularly, to a digital PLL having an auto-gain control.
In an optical disc drive, an EFM (Eight-to-Fourteen Modulation) signal read from a disc needs to be synchronized so that the EFM signal can be further processed. In general, the optical disc drive is operated under a CAV (Constant Angular Velocity) mode and therefore, a channel bit rate of the EFM signal varies as a pick-up head of the optical disc drive moves from an inner track to an outer track or from an outer track to an inner track of the disc. The channel bit rate variation appears in way of phase and frequency variations of the EFM signal. To track the channel bit rate variation of the EFM signal, a phase-locked loop (PLL) is used.
Please refer to FIG. 1. FIG. 1 is a block diagram of a conventional digital PLL circuit 100 used for tracking the EFM signal. The PLL circuit 100 includes a phase detector (PD) 110, a plurality of multipliers 120 and 130, a digital loop filter 140, and a digitally controlled oscillator (DCO) 150. As the pick-up head moves from one track to another track of a disc, phase and frequency characteristics of the EFM signal change, and a multiplier gain G1 of the multiplier 120 and a multiplier gain G2 of the multiplier 130 require adjustments in order to lock the EFM signal. As known to those skilled in this art, the multiplier gains G1 and G2 set in PLL circuit 100 are acquired from a predetermined lookup table. As the pick-up head moves, suitable settings of these gains G1 and G2 will be decided according to the predetermined lookup table.
In a preferred operating condition, utilizing the lookup table to perform a gain control of the multipliers 120 and 130 satisfies the needs of the optical disc drive under CAV mode. However, the pick-up head will fail to read the EFM signal if a disc was not initially recorded properly. Utilizing the lookup table is insufficient to compensate for this fault since the lookup table expects only phase and frequency variations due to a movement of the pick-up head. Therefore, no matter how many times a re-read operation of the disc is performed, the optical disc drive still fails to read the disc under the above condition.